The present invention relates to an amplifier circuit, in particular an amplifier circuit with low leakage power.
Amplifiers that not only release energy but also, at least at times, take up energy at their output give rise to an unnecessarily high leakage power. It is not exclusively the supplied energy that is dissipated, but also additional energy which is withdrawn unnecessarily from the voltage supply. Therefore more elaborate, more expensive measures are required for the dissipation of heat. Furthermore, supply energy is wasted, which is disadvantageous particularly in the case of battery-driven devices.
An example of the uptake of energy by an amplifier via the output thereof is the connection of a reactive, for example capacitive, load to the output of the amplifier. A capacitive load of such a type is, for example, the European ringer load of a terminal device on an analogue subscriber line. Amplifiers that drive a capacitive load have a high leakage power. In the course of the charging process, at least the same amount of energy is dissipated in the amplifier as is transferred into the capacitive load. In the course of the discharging process, in addition to the amount of energy stored in the capacitive load at least twice as much energy is again dissipated in the amplifier. In the most favourable case, i.e. in the course of charging the capacitive load or the corresponding capacitance to the value of the full supply voltage, for each charge-discharge cycle the fourfold value of the energy stored maximally in the capacitive load is dissipated.
A further example of the uptake of energy by an amplifier via the output thereof is the connection of a reactive inductive load, for example an electromechanical or electroacoustic converter, to the amplifier. Also in the case of full-duplex transmission lines such as are employed, for example, in XDSL transmission systems (“Digital Subscriber Line”) for bidirectional transmission of speech and data, at the amplifiers that are connected at both ends of the transmission line with impedance synthesis the uptake of energy occurs via the outputs of said amplifiers.
FIG. 5 shows schematically an amplifier 1 which is connected to a load 3, in particular a reactive load, via its output terminals A1, A2. In the example that is represented, the amplifier 1 comprises two sub-amplifiers 4, 5, the output of sub-amplifier 4 being coupled to output terminal A1 and the output of sub-amplifier 5 being coupled to output terminal A2. The two sub-amplifiers are connected to supply-voltage nodes or supply-voltage terminals V+, V− of the amplifier 1, to which in turn a supply voltage 2 is connected. In the case of an uptake of energy by the amplifier 1 via the output thereof by reason of the reactive load connected thereto, a flow of current occurs from the one output terminal to the other output terminal, this flow of current being routed, in particular, at least partially via the supply-voltage path. Thus, for example, in the case of the uptake of energy via the output of the amplifier 1 a flow of current starting from the output terminal A1 may be directed to the other output terminal A2 of the amplifier 1 via the sub-amplifier 4 and the supply-voltage terminal V−, the supply-voltage source 2, the supply-voltage terminal V+ and the sub-amplifier 5. This flow of current via the supply-voltage path results in the problems previously described, namely, in particular, the fact that in the case of an uptake of energy via the output of the amplifier 1 additional energy, which is withdrawn unnecessarily from the supply-voltage source 2, is dissipated in the amplifier.
In conventional solutions an overheating of the components by the additionally dissipated energy is avoided by appropriate dimensioning of the dissipation of heat from the amplifier 1, for example by providing an appropriately large cooling attachment on the amplifier 1. However, the effort required for these measures is relatively great, and the costs thereof are relatively high.
The object underlying the present invention is therefore to make available an amplifier circuit in which the leakage power of the amplifier circuit can be reduced with low costs and with little effort.